This project seeks to describe the epidemiology of infections due to Legionella pneumophila by a retrospective serologic analysis of paired sera from 4,000 pneumonia patients and compare the frequency of infection and antibody levels with that of basically healthy control groups. The objectives include determination of the community incidence of all four serotypes of Legionella pneumophila pneumonia, by age, sex, calendar year and season, searching for possible clustering in time and place, describing symptomatology of the pneumonia and also searching for evidence of non-pneumonic infections in the community. We will adopt the microimmunofluorescence test using formalinized organisms of all four serotypes in the serologic survey. Preliminary studies indicated that non-heated antigens caused fewer non-specific reactions than those heated. The test apears potentially more type-specific, simple, and efficient. Microagglutination or ELISA test will also be utilized. The study of pneumonia was carried out among some 150,000 members of a prepaid medical care group between 1963 and 1975 and the patients have already been thoroughly investigated for evience of Mycoplasma pneumoniae. Chlamydia trachomatis, influenza A and B, parainfluenza, respiratory syncytial and adenovirus infections. Thus, there is an excellent opportunity to study the relationship of L. pneumophila infection to that of M. pneumoniae or Chlamydia, as well as other viral infections.